Laravel Eloquent Sum of relation's column
Stefan Izdrail
Founder & Senior Architect · 2026-06-29
Title: Calculating Sum of Relation's Column in Laravel using Eloquent
Introduction: In this blog post, we will discuss how to calculate the sum of prices (a column of product) belonging to products associated with a user in a shopping cart application. We will cover the basics of Eloquent relationships and the Laravel query builder for solving this issue without using raw SQL queries. It's essential to understand database tables, relationships, and Eloquent models before we dive into the solution.
Prerequisites:
- A basic understanding of Laravel framework and its fundamentals like routes, controllers, views, and Eloquent models.
- Knowledge of Laravel relationships in Eloquor.
- An idea about the Laravel query builder for performing complex queries.
Body:
To understand this problem, let's first look at the database structure and existing models. We have a `User` model with many-to-many relationship to both `Product` and `Cart`. This relationship can be defined using Eloquent relationships as follows:
```php
// User Model
class User extends Authenticatable
{
// Other attributes...
public function products()
{
return $this->belongsToMany(Product::class);
}
public function carts()
{
return $this->hasMany(Cart::class);
}
}
// Cart Model
class Cart extends Model
{
// Other attributes...
public function user()
{
return $this->belongsTo(User::class);
}
public function products()
{
return $this->hasMany(Product::class);
}
}
// Product Model
class Product extends Model
{
// Other attributes...
public function users()
{
return $this->belongsToMany(User::class);
}
public function carts()
{
return $this->belongsToMany(Cart::class);
}
}
```
Now let's tackle the problem. We want to calculate the sum of the prices of all the products in a user's cart. There are two ways to achieve this:
1. Using Eloquent relationships and the Laravel query builder:
- First, get all the carts related to the current user using `User::with(['carts' => function ($query) { $query->select('product_id', 'price'); }])`. Here, we only retrieve the relevant columns for each product. This reduces the amount of data fetched and improves performance.
- Second, get all the products linked to the carts by calling `$userCartProducts = User::carts()->get()->pluck('product_id');`
- Finally, execute a query using the Laravel query builder where we sum the prices of these products: `Product::whereIn('id', $userCartProducts)->sum('price')`.
Here is an example implementation with code for this approach:
```php
// Controller method to calculate the sum of product prices
public function getUserCartTotalPrice()
{
// Fetch user cart information and only select relevant columns from related products.
$userCarts = User::with(['carts' => function ($query) { $query->select('product_id', 'price'); }])->get();
// Get the product IDs of all the items in the user's cart.
$userCartProductIds = User::carts()->get()->pluck('product_id');
// Calculate the sum of prices of products linked to these carts using Laravel query builder.
$sumOfPrices = Product::whereIn('id', $userCartProductIds)->sum('price');
return view('cart-total-price')->with(['userCarts' => $userCarts, 'sumOfPrices' => $sumOfPrices]);
}
```
2. Using raw SQL queries: If performance is not an issue, you can simply use a custom query to achieve this result without Eloquent relationships or the Laravel query builder. Here's an example using the given database schema:
```sql
SELECT SUM(p.price) AS total_price FROM users AS u
INNER JOIN carts AS c ON c.user_id = u.id
INNER JOIN products AS p ON p.id = c.product_id;
```
Conclusion:
In this blog post, we explored how to calculate the sum of product prices in a user's shopping cart using Eloquent relationships and the Laravel query builder. We presented two approaches - one based on Eloquent and another with raw SQL queries. You can choose the best approach according to your project's requirements and performance needs. Remember that clean and well-structured code is key when working with complex database relationships in Laravel applications.